The present invention relates to an apparatus for developing an electrostatic latent image with a dry-type two-component developer while stabilizing the density of the developed image at all times.
FIG. 8 of the accompanying drawings illustrates an image developing apparatus which is generally employed for carrying out a method of developing an electrostatic latent image using a dry-type two-component developer. The image developing device includes a large toner tank 1 housing various agitating mechanisms such as an agitating roller 2, a feed screw 3, and an agitating separator 4. Toner which has been supplied from a toner hopper 5 is mixed with a carrier and agitated by these agitating mechanisms, and then delivered onto a developing roller 6 serving as a developer carrier, on which the toner is deposited as a magnetic brush layer. The thickness of the deposited developer or magnetic brush layer is limited by a doctor blade 7.
The developing roller 6 includes a sleeve 8 with a pluraltity of magnets 9 disposed therein. At least one of the sleeve 8 and the magnet assembly is rotated in one direction to move the magnetic brush on the circumferential surface of the sleeve 8 in a certain direction. The magnetic brush is brought into contact with a photosensitive body 10 to develop an electrostatic latent image thereon into a visible toner image. After the image has been developed, the magnetic brush is scraped off the developing roller 6 into the toner tank 1. The toner that has fallen into the toner tank 1 is agitated and mixed again by the agitating mechanisms.
The two-component developer is required to be well agitated and mixed for uniform toner density or good toner charging. The conventional apparatus for developing electrostatic latent images using the two-component developer are advantageous in that developed images are of stable quality. However, the various agitating mechanisms are necessary for sufficiently agitating the developer, and a large space is required for defining an agitating passage in which the developer is agitated by those agitating mechanisms. Another problem is that the carrier of the developer is fatigued by the agitation of the developer, resulting in a reduction of carrier durability.
In order to eliminate the drawbacks of the image developing apparatus using the two-component developer, there have been proposed various image developing apparatus in which the developer is not mixed and agitated. One such image developing apparatus is of the self-balanced type as disclosed in U.S. Pat. No. 4,615,606 and includes a charging roller for depositing toner thereon, the charging roller contacting a magnetic brush to supply toner. In another image developing apparatus, a magnetic brush is employed to supply toner to a developing roller for forming a thin toner layer on the developing roller. The former image developing apparatus is however problematic in that the toner on the developing roller has irregular densities because uniform balancing forces cannot be obtained due to irregular charged amounts among toner particles and irregular toner particle diameters. With the latter apparatus, the developer is still required to be well mixed and agitated.
The applicant has filed patent applications on a hybrid-type developing apparatus employing a dry-type two-component developer as a one-component developer (see U.S. patent application No. 016,739, U.K. Patent Application No. 8703853, West German Patent Application No. P3705496.4, and French Patent Application No. 8702278). In this hybrid-type developing apparatus, a resilient brush means such as a magnetic brush or a fur brush on a sleeve is held in contact with a photosensitive drum as a latent carrier, and an image developing region is defined in the area where the magnetic brush contacts the photosensitive drum. A toner supply roller as a toner supply means is disposed in contact with the magnetic brush upstream of the image developing region in the direction of movement of the magnetic brush, and a toner recovery roller as a toner recovery means is located downstream of the image developing region.
The hybrid-type developing apparauts operates as follows: Toner stored in a toner hopper is supplied by the toner supply roller as it is rotated, while being triboelectrically charged thereby, as a thin uniform layer to the magnetic brush under a given toner supplying bias voltage. The magnetic brush to which toner has been supplied is transferred toward the photosensitive drum to develope an electrostatic latent image formed on the drum. After the image has been developed, the magnetic brush has toner density differences or irregularities in pattern corresponding to the image. The residual toner on the magnetic brush is transferred to and recovered by the toner recovery roller under a toner recovering electric bias. According to the arrangements disclosed in U.S. Pat. Nos. 4,347,299 and 4,230,070, a toner recovering bias voltage commensurate with the density of an original, i.e., a toner recovering bias voltage corresponding to a toner consumption, is applied to keep a constant toner supply at all times. In the hybrid-type developing apparatus, however, toner is recovered until the toner density of the magnetic brush is uniformized, irrespective of how toner may be consumed. The toner density differences in the surface layer of the magnetic brush which contributes to image development are thus eliminated to make the toner density uniform. More specifically, after the toner recovery, the magnetic brush contains only the carrier or has a uniform toner density distribution, and is moved away from the toner recovery roller toward the toner supply roller.
In the hybrid-type developing apparatus, the carrier of the developer is not required to be scraped off the sleeve or continues to move on the sleeve at all times. It has been found, however, that in case an ordinary carrier is used as the magnetic brush carrier, carrier particles tend to move progressively toward the opposite ends of the sleeve where the carrier particles are collected at a high density, with the result that the carrier is localized and images will not be developed uniformly.
Such carrier particle localization is caused in the following manner: As shwn in FIG. 9, the sleeve 12 is rotatable with a sleeve shaft 12a made of a nonmagnetic material such as aluminum and houses therein a development magnetic roller 13 having one end rotatably coupled to a bearing 13a supported on the shaft 12a and the opposite end fixed to a a side plate. As indicated by I in FIG. 9, the radial distribution of magnetic flux densities produced on the sleeve 12 by the development magnetic roller 13 is progressively increased from a magnet end and becomes constant beyond about 5 mm from the magnet end. At a magnet end, lines of magnetic force are directed rather axially than radially as indicated by II since the distance between opposite poles is small. As a result, the formed magnetic brush is radially erected at the central area of the development magnetic roller 13 whereas it is directed or inclined axially along the lines of magnetic force at each of the magnet ends.
The magnetic brush at each end of the development magnetic brush 13 is of a common configuration among almost all magnetic rollers. By substantially reducing the distance between magnetic poles through a reduction in the diameter of the development magnetic roller or an increase in the number of magnetic poles, the axial components of the lines of magnetic force are prevented from being increased, and the generation of inclined magnetic brush elements is reduced (it is impossible, in principle, to direct all of the lines of magnetic force radially at each end of a magnetic roller with magnets located inside a sleeve).
A development magnetic roller having a diameter of about 20 mm and alternate 6 to 8 magnetic poles of different polarities still suffers inclined magnetic brush components at each roller end. Therefore, the distance between magnetic poles appears relatively large with such a roller diameter and number of magnetic poles.
Attempts to reduce the pole-to-pole distance would invite a reduction in the magnetic forces of the magnets themselves, thus incapable of retaining carrier particles on the sleeve which would fail to function as a development magnetic roller. One conventional solution has been to seal the ends of the sleeve 12 with resilient members and utilize only the central portion of the magnet.
Where an ordinary carrier is used as the magnetic brush carrier, carrier particles are apt to be scattered away upon rotation of the sleeve 12. The amount of the scattered carrier is increased as the rotational speed of the sleeve 12 goes higher, and is determined by the balancing between magnetically attractive forces and centrifugal forces acting on the carrier particles.
Another problem with an ordinary carrier used as the magnetic brush carrier is that carrier particles do not substantially move between the surface and lower layers of the magnetic brush formed on the sleeve 12. Carrier particles near the sleeve surface are of a high carrier particle density and are prevented from moving by surrounding carrier particles even when the magnetic field to which they are subjected is varied. On the other hand, those carrier particles wich are present near the ends of the magnetic brush are relatively freely movable because of a lower particle density, and can consequently be moved as the magnetic field changes. Due to such different carrier particle behaviors, any exchange of carrier particles between the surface and lower layers of the magnetic brush is highly improbable to occur. Therefore, toner deposited on the surface of the sleeve which serves as support for the magnetic brush would not be removed. An electrically insulative toner layer is thus deposited in covering relation to the surface of the sleeve, changing the electric characteristics thereof. As a consequence, the quality of reproduced images or printed images is impaired.
General image developing apparatus have a means for detecting the remaining amount of toner in a toner hopper. Such a detecting means comprises a mechanical detector disposed in a driver mechanism 32 (FIG. 10) for a toner supply roller 31 as shown in FIG. 8.
More specifically, when there is toner in the toner hopper 5, an agitator gear 33 is rotated by a driving gear 34 to supply toner. A detector flange 36 is integral with an agitator 35 (FIG. 8) in the toner hopper 5, and its rotation is delayed from the agitator gear 33 since the agitator 35 is loaded by the toner in the honner hopper 5. Therefore, a recess 36a defined in the detector flange 36 and a recess 37a defined in a cam 37 integral with the agitator gear 33 are positioned out of alignment with each other, as shown in FIGS. 11(a) and 11(b). As a result, a detecting arm 39 of a toner shortage sensor 38 is not operated and no toner shortage signal is generated.
When the agitator 35 is turned until it is released form the toner load, the detector flange 36 is pulled by a spring 40 acting between the detector flange 36 and the agitator gear 33, and is turned until the recesses 36a, 37a are alinged with each other. Insofar as the toner hopper 5 contains toner, therefore, the detector 36 and the cam 37 repeat the above operation.
In case the toner hopper 5 runs out of toner, the agitator 35 is no longer loaded, and the agitator 35 rotates with the agitator gear 33. The recesses 36a, 37a thus remain aligned under the force of the spring 40. The detecting end 39a of the detecting arm 39 is brought into the recesses 36a, 37a to cause the toner shortage sensor 38 to produce a toner shortage signal, as illustrated in FIGS. 12(a) and 12(b).
The conventional image developing apparatus having such a toner shortage detecting means is complex in construction. In addition, the toner shortage detecting means may fail to operate properly because the relationship between the remaining amount of toner in the toner hopper and the toner load on the agitator is liable to fluctuate.